mix it up
How We Can Incorporate Recycled Plastics in Asphalt Pavement Editor’s Note: This article includes opinions of the author. As a sea turtle conservationist, I want to see the floating island(s) of plastic pollution eliminated from our planet. Contrary to popular belief, we cannot use our roadway system as the new landfill. Even back in 1998-99, a researcher bemoaned the problems with using recycled glass in mix designs during a presentation at the University of Missouri-Rolla, giving a scathing review of Glasphalt. I’ve not forgotten the spirit of his lesson. Throwing trash into our roadway system is not the climate-change solution we want it to be when one considers the health, safety and environmental (HSE) implications alongside the lifecycle cost analysis of a pavement that may or may not perform to our standards for the lifetime that it should. What I mean is the asphalt industry currently designs low-carbon-footprint pavement systems that can last 35, 45, 65 years and beyond (see Sarah Redohl’s excellent report of a Perpetual Pavement Award winner on page 48). How much of the asphalt pavement’s carbon footprint savings do we sacrifice to increased maintenance activities by adding an unverified waste product? In their “Interim Guidelines for the Use of Recycled Waste Plastic in Local Government Road Surfacing Applications,” published July 2021, Austroads’ Azeem Remtulla and Steve Halligan found an increasing interest in using recycled plastics, but found “no independent or comprehensive research has been undertaken into the effect of incorporating recycled waste plastic in asphalt and bitumen including HSE impacts, potential microplastic generation, leaching, fuming, reuse of asphalt incorporating recycled waste plastic and whole of life sustainability.” With this article, we will look at the “whole of life sustainability” concept and use that to consider HSE impacts. We will also look specifically at the idea many environmentalist groups have suggested regarding the leach-
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10 / December 2021
Photo by Naja Bertolt Jensen on Unsplash
ing of microplastics from pavement systems once recycled plastic waste becomes standard. There is a way to guard against that potential for harm, and the industry has already considered how to prevent it. Let me oversimplify the incorporation of recycled plastic waste to make my initial point.
DON’T CROSS THE STREAMS
To incorporate recycled plastic waste in pavements, conveyors and machines powered by coal-fired electricity must sort the plastic streams. In the three-part series “Industry Incorporates Sustainability at Asphalt Production Plants,” which AsphaltPro published in 2019-2020, Malcolm Swanson, P.E., proprietor of e5 Engineers, Chickamauga, Georgia, reminded us that all plastics are polymers, but not all polymers are suited for use in all processes. “There are several types of plastics to be considered,” Swanson shared. “But only two ways of incorporating the plastics into the asphalt. The two methods are ‘wet’ and ‘dry.’” Krishna Srinivasan, president of Sripath Technologies LLC, Mahwah, New Jersey, listed the differentiation of methods as a top priority for the asphalt industry to keep
in mind when incorporating any form of recycled waste plastic in an asphalt mix design. “For dry modifiers, in many cases, the waste plastic functions as a filler,” Srinivasan said. “Even so, workability of the mix is important. The optimum binder content in mixes is defined by looking at density after a certain number of gyrations. If the plastic interferes with the packing, then more binder may be needed, obviously impacting cost of the mix. “For wet modifiers, several factors come into play,” he continued. “The first is the sorting issue—separating out the different plastics and incorporating only those with melting points that are compatible with the processing of the binder. High viscosity in such systems is also an issue. In the case of partially melted plastics, the particle sizes of the unmelted materials may interfere with measurement of key properties needed for verification of the binder grade used in the mixes. “Finally, one of the most dominant issues in waste plastics use in asphalt is the storage stability of the binder incorporating such plastics; often it is quite poor.” Swanson listed for us the waste materials where researchers were focusing their efforts in 2019/2020; where researchers saw the best
